NotificationsController - Handle mising parent notificationsetting

If the parent is missing from base_and_ancestors, we need to gracefully
show a default notification setting

app/finders/user_group_notification_settings_finder.rb

@@ -7,7 +7,9 @@ class UserGroupNotificationSettingsFinder
   end
 
   def execute
-    groups_with_ancestors = Gitlab::ObjectHierarchy.new(groups).base_and_ancestors
+    # rubocop: disable CodeReuse/ActiveRecord
+    groups_with_ancestors = Gitlab::ObjectHierarchy.new(Group.where(id: groups.select(:id))).base_and_ancestors
+    # rubocop: enable CodeReuse/ActiveRecord
 
     @loaded_groups_with_ancestors = groups_with_ancestors.index_by(&:id)
     @loaded_notification_settings = user.notification_settings_for_groups(groups_with_ancestors).preload_source_route.index_by(&:source_id)
@@ -27,6 +29,8 @@ class UserGroupNotificationSettingsFinder
 
     parent_setting = loaded_notification_settings[group.parent_id]
 
+    return user.notification_settings.build(source: group) unless parent_setting
+
     if should_copy?(parent_setting)
       user.notification_settings.build(source: group) do |ns|
         ns.assign_attributes(parent_setting.slice(*NotificationSetting.allowed_fields))

changelogs/unreleased/fix_notificationsettings_page_nonexistent_parent.yml

+---
+title: NotificationsController - Handle mising parent notificationsetting
+merge_request: 41612
+author:
+type: fixed

spec/finders/user_group_notification_settings_finder_spec.rb

@@ -71,6 +71,43 @@ RSpec.describe UserGroupNotificationSettingsFinder do
         end
       end
 
+      context 'when the group has parent_id set but that does not belong to any group' do
+        let_it_be(:group) { create(:group) }
+        let_it_be(:groups) { [group] }
+
+        before do
+          # Let's set a parent_id for a group that definitely doesn't exist
+          group.update_columns(parent_id: 19283746)
+        end
+
+        it 'returns a default Global notification setting' do
+          expect(subject.count).to eq(1)
+          expect(attributes(&:level)).to eq(['global'])
+          expect(attributes(&:notification_email)).to eq([nil])
+        end
+      end
+
+      context 'when the group has a private parent' do
+        let_it_be(:ancestor) { create(:group, :private) }
+        let_it_be(:group) { create(:group, :private, parent: ancestor) }
+        let_it_be(:ancestor_email) { create(:email, :confirmed, email: 'ancestor@example.com', user: user) }
+        let_it_be(:groups) { [group] }
+
+        before do
+          group.add_reporter(user)
+          # Adding the user creates a NotificationSetting, so we remove it here
+          user.notification_settings.where(source: group).delete_all
+
+          create(:notification_setting, user: user, source: ancestor, level: 'participating', notification_email: ancestor_email.email)
+        end
+
+        it 'still inherits the notification settings' do
+          expect(subject.count).to eq(1)
+          expect(attributes(&:level)).to eq(['participating'])
+          expect(attributes(&:notification_email)).to eq([ancestor_email.email])
+        end
+      end
+
       it 'does not cause an N+1', :aggregate_failures do
         parent = create(:group)
         child = create(:group, parent: parent)